Copolymers of monoethylenically unsaturated dicarboxylic acids, monoethylenically unsaturated monocarboxylic acids and monoethylenically unsaturated carboxyl-free monomers are useful as antiscalants, dispersants, incrustation inhibitors in detergents, deflocculants and drilling fluid aids. Several techniques employing aqueous solvent systems have been described heretofore for the preparation of these copolymers, including both batch and continuous processes.
The previous art relating to copolymers of the type involved in the present invention recognized the large differences in the reactivity between dicarboxylic monomers such as maleic acid and monocarboxylic monomers such as acrylic acid. In order to obtain complete or nearly complete polymerization of the maleic acid, batch processes using a maleic acid salt "heel" to which is slowly fed the faster polymerizing acrylic acid monomer were the conventional method of manufacturing these copolymers (U.S. Pat. No. 4,659,793). A more recent continuous process uses a long residence time in a complex series of "cascading" reactors (U.S. Pat. No. 4,725,655).
Previously, copolymer solutions produced according to the batch "heel type" processes suffered from high levels of unreacted dicarboxylic acid which decreased the effectiveness of the copolymer in certain applications. This problem was presumably solved in U.S. Pat. No. 4,659,793 which describes an aqueous batch process for the preparation of copolymers of monoethylenically unsaturated aliphatic dicarboxylic acids, such as maleic acid, itaconic acid, mesaconic acid, fumaric acid, methylene malonic acid, citraconic acid and their monoalkali metal or monoammonium salts, with .alpha.,.beta.-ethylenically unsaturated monomers having carboxyl groups such as methacrylic acid or sulfonic groups such as 2-acrylamido-2-methyl propane sulfonic acid. By using a water-soluble radical generating initiator such a hydrogen peroxide in concentrations from 0.5 to 10% by weight of the total monomer concentration, from 1 to 250 ppm metal ions, such as ferrous or ferric ions, and controlling the pH in the range of 2 to 7, this procedure was presumably successful in reducing the residual unpolymerized dicarboxylic acid level to less than 0.5% while maintaining control of the copolymer molecular weight. The total monomer concentration was from 10 to 70% by weight of the dicarboxylic acid of 4 to 6 carbon atoms per molecule and from 30 to 90% by weight of the ethylenically unsaturated monomers of 3 to 10 carbon atoms per molecule.
In a typical batch procedure described in U.S. Pat. No. 4,659,793, a reactor was charged with the dicarboxylic acid along with the metal ion salt. The reaction mixture was heated from 40.degree. to 150.degree. C. and the initiator and the .alpha.,.beta.-ethylenically unsaturated monomer in an aqueous solution were then added to the heated reaction mixture over 3 to 10 hours. However, because the reaction process begins with the monoethylenically unsaturated dicarboxylic acid already in the reaction vessel and where the monocarboxylic acid is added along with the initiator over a designated time interval, copolymers with compositions that are changing throughout the reaction are synthesized. During the first stages of the reaction the dicarboxylic acid is present in a high concentration leading to the production of a copolymer composed primarily from the dicarboxylic acid. As the monomer content of the reaction solution changes, increasing in concentration of the added monomer, the composition of the initially manufactured polymer changes, increasing in concentration of the added monomer. Similarly, in the final stages of the reaction, since most of the dicarboxylic acid has been polymerized and the added monomer is now present in a higher concentration relative to the dicarboxylic acid, the composition of the copolymer formed at that stage is primarily made up from the added monocarboxylic monomer.
Another disadvantage of the heel method discussed in U.S. Pat. No. 4,659,793 is that to achieve the more desirable low molecular weight copolymers, a higher level of metal ion is needed during the reaction. However, this is ecologically undesirable since the metal remains in the final product and may eventually find its way into the environment.
Another process for the synthesis of these polymers is a continuous cascade process, disclosed in U.S. Pat. No. 4,725,655, which claims to be faster and more economical than a batch process. Described in this patent is the copolymerization of monoethylenically unsaturated dicarboxylic acids containing from 4 to 6 carbon atoms per molecule in concentrations of from 10 to 60% by weight of the monomers, monoethylenically unsaturated monocarboxylic acids containing from 3 to 10 carbon atoms per molecule in concentrations of 40 to 90% by weight of the monomers and optionally carboxyl-free monomers in concentration of not more than 20% by weight of the monomers. This process, which can only be run continuously, begins by charging all the reaction vessels, typically three, with an aqueous solution of a copolymer of monoethylenically unsaturated mono- and dicarboxylic acids prepared by a batch process according to patent application Ser. No. P 3,147,489.6. Starting copolymers of similar composition to those being synthesized are used. The reactors are then heated from 60.degree. to 150.degree. C. and the dicarboxylic acid and at least 40% of the monocarboxylic acid are introduced into the first reactor along with from 0.5 to 6.0% initiator and enough neutralizing solution to neutralize from 20 to 80% of the acids. Additional amounts of monocarboxylic acid and initiator are added to the second reactor or distributed between all the downstream reactors. If a comonomer of a carboxyl-free monomer is used, it can be fed into the first reactor or a combination of the first reactor and the downstream reactors. The total amount of monomer used is from 20 to 70% of the total weight of the aqueous solution. The reaction solution is removed from the last reactor after a residence time of from 0.5 to 8 hours.
Similar to the batch process described above, this process also leads to a polymer with a mixed composition. This is partly due to the fact that the initial charge copolymer was synthesized using a batch process, producing copolymers with a skewed compositions. Also, even though all the monomers can be added simultaneously to the first reactor leading to a more consistent composition, once the copolymer enters the second reactor and only monocarboxylic acid and/or carboxyl-free monomers are being added into this reactor, the composition changes and is composed primarily from the added monomers. The multiple reactors and the equipment required for this process, together with the control problems associated with the process, are thought to be disadvantageous in regard to the efficiency of the process and the consistency of the copolymer composition.
It is the object of the present invention to provide a relatively uncomplicated process capable of yielding a polymer with a more consistent composition in the hope that a more consistent polymer composition would enhance the properties of these copolymers. It is a further object of the present invention to provide a process whereby the molecular weight of the copolymer can be controlled while both the level of residual monomer and the amount of metal remaining in the final product are kept at an ecologically acceptable level.